Cattle grazing white locoweed in New Mexico: influence of grazing pressure and phenological growth stage.

Abstract

Locoweed poisoning generally occurs in early spring when other forage is dormant or in short supply and locoweed is the main green plant available to grazing livestock. The objective of this study was to estimate the amount of white locoweed (Oxytropis Sericea Nut. ex T&G) consumed by cattle, and to determine if cattle graze locoweed because it is relatively palatable, or if they are forced to graze it because of decreasing availability of other forage. Three grazing trials were conducted that corresponded to the vegetative, flower, and pod phenological growth stages of white locoweed. Four cows were used in Trial 1 (vegetative growth stage), and 7 cows were used in Trials 2 (flower stage) and 3 (pod stage). Pastures were fenced for the 10-day grazing trials, so that forage became limited and grazing pressure increased as the trials progressed. Acceptance of white locoweed at the beginning of each trial, when there was adequate forage, would indicate preference. Rejection of white locoweed at the beginning of the trials, followed by increasing consumption as the trials progressed would indicate that grazing pressure was forcing the cows to select white locoweed. White locoweed was readily accepted by 1 cow in the vegetative trial, and by 2 cows in the flower trial (these cows were termed "loco-eaters"). The remainder of the cows (termed "normal") rejected white locoweed in the vegetative and flower trials until the availability of new growth cool n grasses decreased, after which they started to select white locoweed. AD cows rejected white locoweed at the beginning of the pod trial but consumed it as availability of other plants decreased. Regression analysis showed that grazing pressure was positively associated with ingestion of white locoweed (r2 = .46 to .88) by the "normal" cows.